The present invention relates to a method and a system for processing of drilling fluid from a drilling hole in an underwater well to a floating drilling rig or drilling vessel. In particular, the invention relates to processing of drilling fluid before a blow-out valve is connected to the drilling hole and a riser is connected between the drilling hole and the floating drilling rig or drilling vessel.
Today""s demands relating to environmental discharges puts great demands on the operators in the oil industry. For example, some of the operators stipulate that there shall not be any discharge of drilling fluid during drilling. During drilling of a new oil well in the ocean bed, or drilling in an already existing well, large amounts of drilling fluid, which must be treated, are produced. This car be oil-based drilling fluid or water-based drilling fluid, depending on whether the drilling which is being carried out, is top-hole drilling or drilling in the coil zones.
In this application, drilling fluid is meant to be fluids which appear during drilling in a drilling hole, such as cuttings, drilling mud, or other drilling fluids.
In recent years, the environmental threats which the oil industry poses have been given increasingly more focus. The authorities have imposed increasingly stronger demands on care for the environment and have strict rules for discharges from offshore installations, as these can have negative effects on the maritime environment. Today, there are, in the main, strict restrictions with regard to discharges of oil-based drilling mud, and discharges of this type have almost been completely stopped in the Norwegian sector of the North Sea.
In a standard well, in which the following holes are drilled without risers (36xe2x80x3-225 m, 26xe2x80x3-1200 m), more than 340 m2 of cuttings will be produced directly from the well. In addition, there is the drilling mud with its mixture of different chemicals. The Norwegian Pollution Control Authority (SFT) introduced a complete ban on dumping of drilling mud and/or drilling fluid in the Norwegian sector of the North Sea in 1993. This was the start of what is today called slurry-fixing plants, which are able to process the return of fluid to the drilling hole.
Today, most of the fixed installations have such plants, but they are only used for injection of oil-containing waste. The injection is carried out in an annular space between two casings in the drilling hole, normally casings with diameters of around 340 mm and 508 mm (13xe2x85x9cxe2x80x3 and 20xe2x80x3). This is based on a pump rate of about 4000 1/min under drilling of about a 311 mm (12xc2xdxe2x80x3) section and about a 216 mm (8xc2xdxe2x80x3) section.
Water-based drilling fluids are discharged directly to the sea and sink to the ocean bottom, something that creates environmental problems for the maritime life both in the ocean and at the ocean bottom. Discharges of drilling fluids can be carried out with the aid of a pump which is connected on a base at the drilling hole The pump acts as a suction pump to create a negative pressure in a sealing device which is arranged round the drill column in the drilling hole.
Disadvantages with today""s methods are that if the water-based drilling fluid is to be transported up to the drilling rig to be injected into a corresponding well, many problems to which there are no solutions at present arise. For example, during top-hole drilling, there are no maritime risers, i.e. a vertical riser which transports drilling mud from the ocean bottom and up to the drilling platform, and in addition, there is no annular space for injection of the water-based drilling fluid.
U.S. Pat. No. 4,149,603 disclose a system and a method of underwater drilling operation, which returns drilling mud to the surface of the water, without the use of a riser, but after a BOP is installed. The system comprises e mud sump connected to the top of a submerged wellhead and pump means to pump mud through a hose and to the surface.
EP 0290250 discloses a method and apparatus for drilling sub sea wells at large depths, where drilling return mud is pumped to the surface by a centrifugal pump. The apparatus is attached to top of the blow-out preventer stack.
None of the prior art documents discloses methods or apparatuses adapted to be used before a riser is connected and a blow-out preventer is installed on the wellhead.
There is, therefore, a need for a method that can remove discharges of drilling fluid returns at a drilling rig or drilling vessel, and which can be applied in connection with the already existing drilling hole applications both on the ocean bottom and on the drilling rig, before both riser and blow-out preventer is installed. There is also a need for a system to carry out the method according to the present invention.
Advantages with the method according to the present invention are that great savings are achieved by being able to recirculate drilling fluid returns. Full drilling rate is maintained in the different sections, i.e. about 311 mm and about 216 mm (12xc2xexe2x80x3 and 8xc2xdxe2x80x3) sections. Moreover, the environment is spared from unnecessary discharges. A faster slurrification of the drilling fluid which is produced during drilling is also achieved, i.e. faster treatment of the pumpable fluid or mud which consists of a solid material sedimented in a fluid. Less strict demands for the slurry. No wearing of casings will occur, and there is no danger that the casing will be damaged. Drilling fluid is kept away from the template, i.e. the base, and no concrete is used around the template. This gives a clear view for the ROV operator (Remotely Operated Vehicle). A greater injection rate is also achieved. In addition, the drilling fluid can also be stored for later, to be transported away from the floating drilling rig.
In connection with drilling on the ocean bed, drilling fluid is formed around the drilling mould (template). It is normal to use remote controlled underwater vehicles (ROV-xe2x80x9cremote operated vehiclexe2x80x9d) with a camera, to monitor and carry out various operations, and the drilling fluid/mud in the area around the drilling hole orifice represents, therefore, a considerable visual problem. Cuttings are fragments of rocks, which under drilling are brought up with the drilling mud.
The object of the prevent invention is, therefore, to provide a method and a system, which eliminates the abovementioned problems. It is also an object to provide a method and a system of processing drilling fluid return from a drilling hole in an underwater well at a floating drilling rig or a drilling vessel, comprising a sealing device connected to a well head, and a pump module to transport drilling fluid, a treatment plant, or a storage installation, for drilling fluid and possibly an injection pump.
The method, according to the present invention, is characterised in that before a blow-out valve is connected to the well head, the submerged pump module and the sealing device provides an outlet pressure, dependent on the specific weight of the mud and the ocean depth, which is high enough for transportation of the drilling fluid from the drilling hole, through the return line and up to the floating drilling rig or drilling vessel.
The system, according to the present invention, is characterised in that a pump module, which is arranged on the ocean bed and connected to a sealing device, is adapted to transport drilling fluid from the drilling hole on the ocean bed, via a return line, to a treatment plant, or a storage installation, on the floating drilling rig or drilling vessel.
Preferred embodiments of the method, according to the present invention, are specified in that the pump nodule placed on the ocean bed and the sealing device provides an outlet pressure, dependent on the weight of the mud and ocean depth, which is high enough to transport drilling fluid from the drilling hole, through the return line and up to the floating drilling rig or drilling vessel. The drilling fluid is transported through the return line and to the existing line (flow-line) on the floating drilling rig or drilling vessel for further transport to the treatment plant or storage installation. After the cuttings is treated, using a method that may be known previously, on the floating drilling rig or drilling vessel, the treated cuttings is injected, with the aid of a high-pressure pump, into a second drilling hole provided on the ocean bed, or in an adapted annular space in the first drilling hole.
Preferred embodiments of the system are characterised in that the submerged pump module and the sealing device, before a blow-out valve is connected to the well head, are adapted to provide an outlet pressure which is high enough for transportation of the drilling fluid from the drilling hole, through the return line and up to the floating drilling rig or drilling vessel.
The pump module placed on the ocean bed comprises a number of pumps to provide the necessary pressure, such as a centrifuge and/or a friction pump possibly connected in series, where the pump, or pumps, is driven by a submerged electric motor which is connected to the pump or pumps.
A preferred embodiment, according to the present invention, shall now be described with reference to the enclosed figures. It must be understood that this example is not limiting and that other and further modifications may be carried out within the scope of the claims.
FIG. 1 shows a principle of the method and the system for processing of a drilling fluid according to the present invention.
FIG. 2 shows a section of an injection well according to FIG. 1.